The ever-increasing quantity of printed information being created, particularly by modern computer systems, has made it necessary to devise typewriting machines which operate at extraordinarily high speeds. One of the most successful types of such machines is a dot matrix printer in which a printing head contains a plurality of electromagnetically actuated wires capable of printing a matrix of dots with their ends to form visually perceivable characters. A significant advantage of of this type of machine is that a single printing head can be used to print an endless variety of letters and characters in a number of languages, such as, for example, English and Chinese.
U.S. Pat. No. 4,389,128 granted June 21, 1983 to K. Asano, et al., for "Print Head For A Dot Matrix Printer" discloses what can be termed a state-of-the-art print head of the type with which the present invention is concerned. The Asano et al. print head has a plurality of radially arranged wire actuating armatures which are maintained in a non-print position by a magnetic structure including an annular permanent magnet. The individual armatures are spring biased to move to their print positions when individual electromagnets associated with the armatures are energized to cancel out the flux from the permanent magnet. The armatures are radially disposed in a circular array and have print wires affixed to their moveable inner ends. The moveable ends of the armatures are disposed in a circle. The other, or printing, ends of the printing wires are positioned in a closely packed array, usually consisting of one or two upright rows.
There is considerable demand for dot matrix printer heads capable of printing high quality characters, i.e., characters which are so formed that the human eye cannot detect the fact that they have been formed from a plurality of individual dots. To achieve this quality of printing, the print head may be equipped with as many as twenty-four print wires. This means that the print head must also contain an equal number of armatures and electromagnets. In order to hold down the size and weight of the print head--which must be done to reduce the inertia of the head,--the wires, the armatures and the electromagnets are all made quite small and are closely compacted within the print head. There is a physical limitation, however, as to how close together the moveable ends of the armatures can be placed because of the bulk of the armatures themselves. Consequently, in prior art print heads, such as that disclosed in the Asano et al. patent, the moveable ends of the armatures and the driven ends of the print wires associated therewith are disposed in a circle which is considerably larger in diameter than some of the dimensions of the array in which the other, or printing, ends of the wires are disposed. This means that the print wires must possess a curvature from their ends to their printing ends and this curvature usually varies from wire to wire.
Dissimilarities between different ones of the wire-armature assemblies within the print head are undesirable because this can cause different printing wires to behave differently from other printing wires in the head during operation of the head. When one considers that the slender print wires must move from a rest position to an impact position and return to the rest position within a time interval of one millisecond or less while providing an impact of sufficient force to print a clearly legible dot without any bounce either on the printing surface or at the rest position, it can readily be appreciated that substantial variations in length and curvature of the individual print wires or in the length or configuration of their armatures can cause the head to print some dots differently from other dots. This "erratic" performance can be expected of some print heads constructed as taught in the Asano et al. patent.
Similar behavior is to be expected of print heads constructed as taught in U.S. Pat. No. 4,225,250, granted Sept. 30, 1980 to R. E. Wagner et al. for "Segmented-Ring Magnet Print Head." In an attempt to bring the position of the driven ends of the print wires into closer correspondence with the positions of the printing ends of these wires, Wagner et al. associate different wires with armatures of different lengths. It should be obvious that an armature which is of different size and mass than its neighbor is likely to have different performance characteristics as well.
A somewhat different print head construction is disclosed in U.S. Pat. No. 3,690,431, granted Sept. 12, 1972 to R. Howard for "Print Head Assembly Containing Solenoids." In the construction disclosed in that patent, various guide members are provided for the print wires to control the behavior of the wires during operation, but because of the relative disposition of the driven ends of the wires in relation to the printing ends of the wires, different wires have different lengths and different curvatures and, therefore, different operating characteristics.
There continues to be a need, therefore, to improve the operating and performance characteristics of high speed print heads.